This invention relates to electroplating aluminum articles and is of particular advantage in electroplating aluminum stock such as strip, rod or wire.
The purposes of electroplating aluminum surfaces vary in a manner generally corresponding to the production of other plated metals, but are often related to the advantages, such as light weight, electrical conductivity of the underlying aluminum, and the desirability of having a finish of a different metal, e.g., for appearance or hardness or corrosion resistance, or the like. One particular reason for electroplating aluminum stock, such as wire or other conductors, is to obviate the electrical contact resistance normally due to oxide. Electroplating with tin, for example, can avoid the formation of a high-resistance, surface film of aluminum oxide which impairs achievement of best electrical conduction to another metal object in contact with the aluminum articles. As will be understood, electroplating of other aluminum articles may be advantageous where a hard, bright, non-corrosive finish is desired, yet lightness of weight is important, as for designing exterior parts of vehicles to save energy by reducing weight.
One known technique for plating aluminum, for instance with tin, has involved the successive steps of degreasing, etching, de-smutting, and then steps such as immersion tinning, a bronze strike, possibly a step of acid conditioning, and finally the actual tin plating itself. The bronze strike and the final tin plating are the only electrolytic steps. Similar steps are used for plating aluminum stock with other metals.
More specifically, one procedure has involved not only a degreasing, but an etching in caustic solution, i.e., a step of purely chemical treatment, followed by a rinse, then an acid dip to remove so-called smut from the surface and thereafter another rinse, before the usual pre-plating treatment of treatments, such as a zincate or stannate immersion. The objectionable features of this technique are that the chemical caustic for cleaning tends to roughen the surface and thereby impair its usefulness when ultimately plated, especially for a bright trim product (e.g., chromium-plated), or the like. The chemical treatment also leaves a smut on the surface which is objectionable for subsequent plating adherence. Hence, a special acid dip is employed to remove the smut. Furthermore, the rinses necessary for both the caustic chemical treatment and the acid de-smutting treatment add complexity to the series, requiring time and equipment, create loss by chemical dragout and cause further disposal problems for the effluents from the system.
In a related patent application of Cooke, Hodgson and Sasaki, for Electroplating Aluminun Stock, Ser. No. 578,324, filed May 16, 1975, now abandoned a procedure for electrolytic cleaning of aluminum stock prior to electroplating is described. The specific cleaning step disclosed and claimed, and the examples of it, involve an acid elctrolytic treatment, particularly utilizing a mixture of concentrated acids, e.g., mixtures of phosphoric acid with sulfuric or nitric acid, or a mixture of sulfuric and chromic acids, at relatively high temperatures. These operations, wherein the metal is anodic, have been found effective in producing a clean, smooth surface, without smut and with an avoidance of oxide film that would be troublesome for electroplating of electrical conductors. The cited application also mentions the possible alternative of an electrolytic alkaline cleaning, but no specific treatment of special value was disclosed, nor any example thereof, nor was any actual chemical solution for such purpose described or named.